Many image capture devices use either a charged coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device to capture the image. Both types of devices are typically sensitive to light across the entire visible spectrum. When a device is used to captures a color image, the device is typically overlaid with a pattern of red, green and blue color filters. One common pattern used to arrange the color filters across a device is a Bayer pattern. In this application, a CCD will be used to represent both a CCD and a CMOS type of device.
The Bayer filter pattern is a repetitive grid of red, green, and blue sub-filters overlaying the CCD array (sec FIG. 1). Adding a Bayer filter transforms the array from an array that is equally sensitive to a wide spectrum of light, into an array that is individually sensitive to red, green, and blue wavelength bands. It enables the capture of color scene in three colors (red, green and blue). It is a de facto element of most color digital cameras. The image resulting from a Bayer filter is a color mosaic of red, green and blue, pixels.
There are times when it is desirable to capture an image as a grayscale image, instead of capturing the image in color. For example, when capturing an image of a printed document a grayscale image may be more useful. Unfortunately having a second CCD without color filters, is costly. Therefore most color capture devices convert the captured color image into a grayscale image when a grayscale image is desired.
Converting a color image into a grayscale image can be done using a number of different techniques. One technique is Green bi-linear interpolation. This is a somewhat low complexity solution that is susceptible to aliasing. It also yields a lower resolution grayscale image because it ignores the information in the red and blue pixels. Other techniques may provide a high quality grayscale image, but may require large amounts of computation, have large storage requirements, or require significant time to complete.